The invention relates to an X-ray image intensifier tube which includes an entrance screen with a layer of luminescent material provided on a substrate as well as a photocathode, and also includes an electron-optical system for imaging photoelectrons, to be emitted by the photocathode, on an exit screen of the tube.
An X-ray image intensifier tube of this kind is known from U.S. Pat. No. 3,825,763. The entrance screen of an X-ray image intensifier tube described therein includes a layer of luminescent material which consists mainly of CsI whereto an activator is added, for example from 0.1 to 1.0 percent by weight of Na or Ti.
X-rays intercepted by this luminescent layer are at least partly converted into luminescent light. The luminescent light releases photoelectrons from a photocathode. The photoelectrons are accelerated and imaged on an exit window where they form a light image in the customary manner. In such tubes a comparatively high brightness intensification is obtained notably by acceleration of the photoelectrons to, for example from 25 to 30 kV. In other types of X-ray image intensifier tube the brightness intensification is notably achieved by photoelectron multiplication, for example by means of a channel plate multiplier.
In known tubes foggy images are formed because secondary X-rays which are released from the luminescent material and which are not readily absorbed by the luminescent material can spread across a comparatively large part of the layer of luminescent material and generate luminescent light as yet. On the other hand, due to the small probability of interception of this radiation by the luminescent material, this secondary radiation contributes only little to the production of luminescent light for which the photocathode is sensitive. Consequently, part of the radiation energy is lost for the imaging process.